1. Field of the Invention
This invention relates to an optical disk and an apparatus for reproducing information from the optical disk.
2. Description of the Prior Art
FIG. 1 illustrates a construction of a known optical disk on which audio information and/or video information is recorded. In FIG. 1, reference numeral 93 denotes substrate and reference numeral 94 denotes protection layer. Information to be recorded is converted into signals, the value of which corresponding to pit lengths used as parameters, and the signals are recorded on information recording surface 95 as information pits 91. In reproduction, a laser light is converged on information pits 91 to form a light spot LS having a predetermined diameter, and reflected and diffracted light beam is detected by photodetector, such as photodiode, to produce electric signal. The electric signal is subjected to a conversion reverse to the conversion carried out in recording process to obtain original audio information or video information recorded on the disk 92. As examples of such an optical disk, Compact Disk (CD) and Laser Disk (LD) are known.
As described above, as to a known optical disk, reading light beam is irradiated on disk and recorded information is read out based on variation in quantity of reflected light according to presence or absence of information pits. Information recording density of those optical disks are defined and limited by track pitch. As shown in FIG. 2, track pitch P.sub.1 is an interval between neighboring tracks which define center-lines of arrays of information pits 91. Recently, in order to increase recording density of optical disk, various measures for reducing track pitch have been developed and proposed.
However, when track pitch is simply reduced, to a value P.sub.2 smaller than conventional value P.sub.1 (P.sub.2 =P.sub.1 /2) for example, as illustrated in FIG. 3, light spot LS of laser light covers not only target information pit 91A to be read-out but also information pits 9lB and 91C on neighboring tracks. This trial is hopeless in practice because reproduced signal includes crosstalk (intermixture of signals from neighboring tracks) too much. Namely, in order to reduce crosstalk, track pitch must be large enough compared with a diameter of light spot LS. This is a bar against increasing recording density.
In the above view, in order to improve recording density, various methods for reducing diameter of light spot of laser beam has been proposed. In general, radius W of light spot LS is expressed as follows: EQU W=.alpha..times..lambda./NA (.alpha.:constant).
Here, NA is numerical aperture of objective lens, and .lambda. is wavelength of laser light. Accordingly, in order to reduce diameter W of laser light, wavelength .lambda. should be reduced, or alternatively numeral aperture NA should be increased. However, improvement of wavelength .lambda. and numerical aperture NA has technological limitation at present, and hence it is hopeless to improve recording density greatly.